The invention relates to a method for limiting the switching current of a switching device and therefore limiting the power dissipated by the switching device at turn-on time, and further returning energy to a power receiving circuit when the switching device disconnects from the power source.
During the turn-on time for a switching device, which may include, but is not limited to, an FET or bipolar transistor, wasteful transition power is dissipated during a transition turn-on time. Thus, the longer the transition turn-on time lasts, the more wasteful power is dissipated.
FIG. 1 shows, for example, a MOSFET switching device 10 which switches a voltage from +V to the input of inductor 20 in a simple buck regulator. It is known to provide a buck regulator, which includes an inductor 20, capacitor 30 and freewheel diode 50 and switching device 10 for producing a first regulated output voltage across the capacitor 30 from a pulsed input supply. Controller 60 is provided to control the duty cycle of the switching device 10. During the transition turn-on time required for the switching device 10 (which may be, but is not limited to, an FET or bipolar transistor), transition power is dissipated, and the longer the transition, in relation to the repetition period of the transition, the more power is dissipated, as further illustrated in this example.
More particularly, the disadvantageous example embodiment of FIG. 1 shows a buck regulator which utilizes a switching device 10, which is used to switch the input of inductor 20 to the input voltage +V for a period of time, and then to disconnect inductor 20 from +V for a subsequent period of time. If switching device 10 has to switch 15A (i.e., 15 amps), the power being switched will be given by the formula 15xc3x97+V. If +V were 20V, then the power switched would be 300 watts, since (15A)(20V)=300watts. That is, as an example, when 15A are transmitted from source +V, and when transistor 10 is turned on, the left side of inductor 20 charges up to +V, which is the voltage level from the power source. Current begins to increase through inductor 20 and capacitor 30 is charged up. In a stable state, the charge on capacitor 30 rises above the output voltage and then back down below the output voltage, producing a ripple. In this example, then, the transistor 10 would be immediately switching 15A at turn-on time, and therefore would be wasting a significant amount of power during turn-on transition time.
According to the invention, a method, including the steps of limiting a current sent from a power source to a switching device, and system are provided for limiting a switching current in a switching device during the transition turn-on time of the switching device by disposing an inductor device in series between the power source and the switching device, and returning flux energy stored in the inductor device to a power receiving circuit at a turn-off time of the switching device.